Osteolysis and aseptic loosening secondary to bearing surface wear remain the greatest limitation to longevity of contemporary total hip arthroplasty (THA). To-date, most effort in reducing wear has been focused on average or cohort-wide assessments, rather than on the small minority of patients at the high end of the wear spectrum who are at greatest risk for loosening. These outlier wear situations need to be much better understood if revision rates are to be lowered. Contemporary trends in THA design and surgeon preference attest that, to most of the orthopaedic community, 3rd body acceleration of polyethylene wear is only a nebulous theoretical abstraction. By contrast, there are immediate attractions (e.g., versatility, surgical convenience, economy of inventory) for adopting 3rd-body-prone design features, such as modularity and biological fixation. Since 3rd body debris seems likely to remain a fact of life for the foreseeable future - increasingly, if present trends continue - it seems reasonable to explore steps to reduce the harm it sometimes does. Understanding the mechanisms by which the damage occurs is necessary if surgeons and designers are to take action accordingly. Our central paradigm is that forestalling 3rd body acceleration of polyethylene wear depends on avoiding scratch production in kinetically critical femoral head regions. Using a new finite element formulation, supported by clinical wear rate measurements, we propose to elucidate direct mechanistic relationships between wear front abnormality, wear rate elevation, and localized femoral head roughening (Aim 1). We suspect that the primary determinant of the severity of wear acceleration due to a given debris burden lies in whether or not particulates are able to gain access to the bearing surface in such a manner as to scratch specific head regions upon whose roughness UHMWPE cup wear most sensitively depends. We propose to identify those most-critical head roughening regions (Aim 2), and to elucidate a novel mechanism - subluxation and fluid convection - by which 3rd body particles plausibly can gain access (Aim 3). We hope to demonstrate that this access route is amenable to influence by modification of surgical technique or by component design changes. Finally, while highly-crosslinked polyethylenes appear to offer dramatic performance improvement under relatively favorable tribologic conditions, it remains to be seen (Aim 4) how well that apparent improvement carries over into the adverse situation of 3rd body challenge.